Dutch elm disease is caused by two fungus species (Ophiostoma ulmi and Ophiostoma novo-ulmi). The fungi and the disease were introduced into North America from the Netherlands in the 1930s. Millions of mature elm trees have died from the disease since that time. Dutch elm disease became widespread in Illinois by 1959. It continues to attack newly planted elms. The fungus is primarily spread from infected to healthy trees by bark beetles. The disease can also be transmitted through interlaced roots of closely growing trees. The fungus blocks water-conducting tissues and can kill a young tree within a few weeks.